1. Field of the Invention
This invention relates to zoom lenses of the rear focus type and, more particularly, to high range, large relative aperture zoom lenses of the rear focus type with a long back focal distance to be used in photographic cameras, video cameras, or cameras for broadcasting. Still more particularly, it relates to such zoom lenses which have a range of about 10 and an F-number of about 1.6.
2. Description of the Related Art
For a zoom lens in a photographic camera or video camera, the focusing method of moving other than the front or first lens unit has been previously proposed. The so-called rear focus type then has found its use in many zoom lenses.
In general, as compared with the type in which the first lens unit is used for focusing, the rear focus type has surpassing advantages that the effective diameter of the first lens unit gets smaller, that it becomes easier to minimize the bulk and size of the entire lens system, that a close-focusing capability, especially a super close-focusing one, is readily built there into, and that, because the focusing lens unit is relatively small in size and light in weight, a weaker driving power for that lens unit suffices, which enables the focus adjustment to speed up.
A zoom lens that embodies such features of the rear focus type is exemplified in Japanese Laid-Open Patent Application No. Sho 63-44614, comprising, from front to rear, a first lens unit of positive refractive power, a second lens unit of negative refractive power for varying the focal length, a third lens unit of negative refractive power for compensating for the shift of the image plane with zooming and a fourth lens unit of positive refractive power. That is, in application to the so-called 4-unit zoom lens, the third lens unit is made movable for focusing. This zoom lens has, however, a tendency to increase its physical length, because there is need to create a surplus space for allowing the third lens unit to move only for focusing purposes.
Another zoom lens in Japanese Laid-Open Patent Application No. Sho 58-136012 has its zooming section constructed with three or more lens units, one of which is used for focusing.
In Japanese Laid-Open Patent Application No. Sho 63-247316, a first lens unit of positive refractive power, a second lens unit of negative refractive power, a third lens unit of positive refractive power and a fourth lens unit of positive refractive power are arranged in this order from the object side, wherein the second lens unit moves axially to vary the focal length, while simultaneously moving the fourth lens unit to compensate for the image shift. Focusing is performed by moving the fourth lens unit.
In Japanese Laid-Open Patent Application No. Sho 58-160913, a first lens unit of positive refractive power, a second lens unit of negative refractive power, a third lens unit of positive refractive power and a fourth lens unit of positive refractive power are arranged in this order from the object side, wherein the first and second lens units move axially to vary the focal length, while simultaneously moving the fourth lens unit to compensate for the image shift. One or two of these movable lens units for zooming are used for focusing.
Japanese Laid-Open Patent Applications Nos. Sho 58-129404 and Sho 61-258217 show so-called 5-unit zoom lenses comprising, from front to rear, a first lens unit of positive refractive power, a second lens unit of negative refractive power, a third lens unit of positive refractive power, a fourth lens unit of positive refractive power and a fifth lens unit of negative refractive power. In this type of zoom lens, the fifth lens unit or a plurality of lens units including the fifth lens unit are used for focusing. In Japanese Laid-Open Patent Application No. Sho 60-6914, there is a unique proposal for a 5-unit zoom lens formed likewise as to the above, but having a property that for an objects at a particular finite distance, the focusing lens unit takes a constant axial position independently of any focal lengths.
In general, by employing the rear focus type in the zoom lens, the above-described advantages are obtained. Among these, the feasibilities of improving the compact form of the entire lens system, of doing quick and easy focusing, and further of readily affording the close-focusing capability are advantageous.
However, on the other hand, the range of variation of aberrations with focusing is increased rapidly. So, a very difficult problem comes to arise when the requirements of maintaining good stability of high optical performance throughout the entire focusing range and of limiting the size of the entire lens system to a minimum are to be fulfilled at once.
Particularly for a large relative aperture, high range zoom lens, the difficulty of obtaining high optical performance throughout the entire focusing range as well as throughout the entire zooming range increases rapidly.
It is also considered that the trend up to now in the field of art of video cameras for public use is that most of their zoom lenses have been adapted for the single chip type image sensor. In this case, the color separation prism and associated parts therewith are not used, which are prerequisite mainly to the lens system of the multiple-chip type for professional use. For this reason, the back focal distance is taken relatively short. In the case of the multiple-chip type, because of the necessity of disposing the color separation prism behind the photographic lens, it is required that the back focal distance be made considerably longer than that for the zoom lenses of the single chip type in the popular video cameras.